Method of decomposing chlorohydrocarbons

ABSTRACT

Exhaust gas containing a chlorohydrocarbon having 1-10 carbon atoms and molecular oxygen is contacted with chromium oxide or a boehmite supported platinum at an elevated temperature to decompose the chlorohydrocarbon to carbon dioxide, water, hydrogen chloride and free chlorine.

This is a division of application Ser. No. 469,804, filed May 14, 1974,now U.S. Pat. No. 3,972,979.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of decomposing a chlorohydrocarbon byoxidative contact with a catalyst.

2. Description of the Prior Art

Exhaust gases from industry have been oxidatively treated to reduce airpollution. It is known that catalytic oxidation using a noble metalcatalyst, such as platinum and palladium or an oxide catalyst, such asoxides of vanadium, chromium, manganese, iron, cobalt, nickel, copper,molybdenum and tungsten, is effective in treating exhaust gasescontaining carbon monoxide and hydrocarbons. However, an industrialmethod of catalytic oxidation of exhaust gases containingchlorohydrocarbons is not known, because conventional oxidizingcatalysts are easily poisoned by halogen or sulfur compounds. A needexists therefore, for effective catalysts which are not poisoned byhalogen compounds.

SUMMARY OF THE INVENTION

Accordingly, one object of the invention is to provide a method ofdecomposing chlorohydrocarbons, with high efficiency, by oxidation usinga catalyst which maintains high catalytic activity for a long period oftime.

This and other objects as will hereinafter become more readilyunderstood by the following description can be attained by contact of agas containing a chlorohydrocarbon of 1 - 10 carbon atoms and molecularoxygen at an elevated temperature. The catalyst used in this inventionis chromium oxide or a boehmite supported platinum. When the chromiumoxide catalyst is used, the chlorohydrocarbon should preferably contain1 - 6 carbon atoms.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The gas to be treated by the oxidative decomposition method of theinvention contains a chlorohydrocarbon preferably having 1 - 10 carbonatoms, such as saturated chlorohydrocarbons, e.g., methyl chloride,ethyl chloride, dichloromethane, dichloroethanes; unsaturatedchlorohydrocarbons, e.g., chloroethylene, dichloroethylenes; aromatichydrocarbons, e.g., chlorobenzene, or the like: the gas can containother oxidizable compounds together with the chlorohydrocarbon and theseoxidizable compounds can be decomposed at the same time in accordancewith the method of the invention.

Suitable catalysts include at least one catalyst selected from the groupof chromium oxide and a beohmite supported platinum. Chromium oxide andboehmite supported platinum maintain high catalytic activity for a longperiod of time in the oxidative decomposition of chlorohydrocarbon.

The chromium oxide catalyst will be described first. The chromium oxidecatalyst can be prepared by using any chromium compound, such aschromium nitrate, chromium chloride, chromium hydroxide, chromicanhydride. The chromium oxide can be supported on a carrier. Suitablecarriers include alumina, silica, silica-alumina, mullite, cordierite,or the like. The carrier can be spherical, cylindrical, honeycomb, orrelated shape. Suitable honeycomb carriers are preferably 50 - 500 mm inlength, more preferably 100 - 400 mm, with porosity of 0 - 90%,preferably 10 - 70%. Suitable spherical or cylindrical carriers arepreferably 0.5 - 20 mm in length or diameter, more preferably 1 - 10 mm,with porosity of 0.5 - 70%.

The chromium oxide can be prepared in any desired shape by thermallydecomposing the chromium compound followed by calcining to form thecatalyst. When a carrier is used, the chromium compound may be supportedon the carrier by a conventional method, such as impregnation,precipitation, or the like and then thermally decomposing and calciningthe chromium compound. The quantity of chromium oxide supported on thecarrier is 0.5 - 80% by weight, preferably 3 - 50% by weight.

The platinum catalyst supported on boehmite will now be illustrated. Theboehmite carrier can be supported by other carriers, that is, theplatinum must be supported on boehmite but the remainder of the catalystneed not be boehmite. A suitable carrier can be prepared by coatingboehmite onto another carrier, which may be spherical, cylindrical,honeyconical or other like shape. The platinum can then be supported onthe carrier by any conventional method, such as by dipping a platinumcompounds such as chloroplatinic acid, etc., into an aqueous solutionand then reducing the compound with or without drying or calcining withhydrogen, hydrazine, formalin, sodium borohydride, or the like. Thequantity of platinum supported on the carrier is preferably 0.03 - 5% byweight.

According to the methods of this invention, the chromium oxide catalystand the boehmite supported platinum catalyst can be combined. When bothtypes of catalysts are mixed and placed in a catalyst bed, a relativelycomplicated operation is required for separating and recovering theplatinum component from the waste catalyst. Accordingly, a series ofcatalyst beds filled with each type of catalyst alone is preferable. Thecatalyst bed filled with the chromium oxide catalyst is preferablyplaced in front of the line and the catalyst bed filled with theboehmite supported platinum catalyst in the rear of the line.

By the method of this invention, the chlorohydrocarbon present in thegas can be effectively oxidized and decomposed by contacting thecatalyst bed with the gas containing the chlorohydrocarbon and themolecular oxygen, at an elevated temperature and at atmospheric orhigher or lower pressure. The temperature of the catalyst bed ispreferably kept at 200°-500° C., since catalytic activity will bereduced at higher temperatures and an insufficient reaction velocitywill result at lower temperature.

The gas containing the chlorohydrocarbon can contain other oxidizablecompounds, such as hydrogen, carbon monoxide, hydrocarbons, etc. Theconcentration of the oxidizable compound will depend upon the heat ofreaction and the catalyst bed is then kept at a suitable temperature.When the concentration of oxidizable compound is too low, the gas isheated before the reaction. On the contrary, when the concentration ofoxidizable compound is too high, the heat of reaction is removed bycooling with a cooling tube inserted in the catalyst bed. Accordingly,the concentration of oxidizable compound is preferably 10 - 10,000 ppmwith the chromium oxide catalyst and 10 - 50,000 ppm with the boehmitesupported platinum catalyst. The space velocity of the gas is preferably5000 - 300,000 hr.sup.⁻¹, and can be selected depending upon theconcentration of oxidiziable compounds at the inlet or the outlet.

Having generally described the invention, a more complete understandingcan be obtained by reference to certain specific examples, which areincluded herein for purpose of illustration only and are not intended tobe limiting unless otherwise specified.

EXAMPLE 1

A reactor having a diameter of 20 mm and height of 300 mm, was filledwith 5 - 20 cc of one of the following catalysts. Air containing 500 ppmeach of ethylene and a chlorohydrocarbon listed in Table 1, was passedthrough the reactor under the reaction conditions listed in Table 1 tocarry out the reaction. The results are shown in Table 1. No change ofcatalytic activity was noted after 10 days from the initiation of thereaction.

Preparation of Catalyst 1

A γ-alumina (diameter of 4 mm and length of 4 mm) carrier was dippedinto an aqueous solution of chromic acid of concentration 252 g/l (Cr₂O₃). The carrier was removed and dried at 100° C. for half a day andcalcined at 400° C. for 2 hours in air to obtain Catalyst 1 whereinchromium oxide is supported on the γ-alumina carrier to the extent ofabout 5% by weight as Cr.

Preparation of Catalyst 2

The same type of γ-alumina as used above was dipped into an aqueoussolution of chromium nitrate of concentration 1000 g/l. The carrier wasremoved and treated with 4N-aqueous ammonia solution. The product wasdried and calcined as set forth in the preparation of the Catalyst 1, toobtain the Catalyst 2.

Preparation of Catalyst 3

A silica-alumina carrier (diameter of 4 mm and length of 4 mm) composedof 30% alumina and 70% silica was dipped into an aqueous solution ofchromic acid of concentration 500 g/l, and was treated as set forth inthe preparation of the Catalyst 1 to obtain Catalyst 3, in whichchromium oxide is supported on the silica-alumina carrier to the extentof 10% by weight as Cr.

                                      Table 1                                     __________________________________________________________________________                    Reaction conditions                                                                     Decomposition (%)                                                   space     halo-                                                               velocity                                                                           temp.                                                                              hydro-                                              Catalyst                                                                            Chlorohydrocarbon                                                                       (hr.sup.-.sup.1)                                                                   (° C)                                                                       carbon                                                                             ethylene                                       __________________________________________________________________________    Catalyst 1                                                                          methylchloride                                                                          40,000                                                                             400  63   60                                             Catalyst 1                                                                          chlorobenzene                                                                           60,000                                                                             350  98   69                                             Catalyst 2                                                                          dichlorometh-                                                                           40,000                                                                             350  55   58                                                   ane                                                                     Catalyst 3                                                                          chloroethylene                                                                          80,000                                                                             350  31   55                                             Catalyst 3                                                                          chloroethylene                                                                          40,000                                                                             400  75   79                                             __________________________________________________________________________

Reference 1

A reactor having a diameter of 20 mm and height of 300 mm was filledwith 5 - 20 cc of the following catalysts. Air containing 500 ppm eachof ethylene and methylchloride was passed through the reactor at 400° C.at a space velocity of 40,000 hr.sup.⁻¹, to carry out the reaction. Theresults are shown in Table 2.

Preparation of Catalysts 4 - 8

Each of the catalysts was prepared by the method set forth in thepreparation of Catalyst 1 except that an aqueous solution of a nitratewas used instead of chromic acid: cobalt nitrate in Catalyst 4; ferricnitrate in Catalyst 6; copper nitrate in Catalyst 7; nickel nitrate inCatalyst 8. Each catalyst contained metal oxide supported on the carrierto the extent of 5% by weight as metal.

Preparation of Catalyst 9

The catalyst was prepared by the method set forth in the preparation ofCatalyst 1 except that an aqueous solution prepared by dissolvingvanadium (V) oxide in oxalic acid was used instead of the aqueoussolution of chromic acid.

Preparation of Catalyst 10

The catalyst was prepared by the method set forth in the preparation ofCatalyst 1 except that an aqueous solution of ammonium molybdate wasused instead of the aqueous solution of chromic acid.

                  Table 2                                                         ______________________________________                                        Catalyst         Decomposition (%)                                            ______________________________________                                                 metal                                                                         supported                                                                             methyl chloride                                                                             ethylene                                       ______________________________________                                        Catalyst - 4                                                                             Co        35            27                                         Catalyst - 5                                                                             Fe        30            12                                         Catalyst - 6                                                                             Mn        27            26                                         Catalyst - 7                                                                             Cu        25            30                                         Catalyst - 8                                                                             Ni        19             3                                         Catalyst - 9                                                                             V         12            15                                         Catalyst - 10                                                                            Mo         6             1                                         ______________________________________                                    

EXAMPLE 2

An aqueous solution of sodium hydroxide was added to an aqueous solutionof aluminum nitrate to precipitate an amorphous gel. The product wastreated at about 200° C. in an autoclave to obtain boehmite (containinga part of the gel). The boehmite was formed into a spherical carrierhaving diameter of about 6 mm. The boehmite was also kneaded with waterto form a slurry. A honeycomb carrier made of α-alumina was dipped intothe slurry to coat boehmite onto the surface of the α-alumina. Theproduct was calcined at 300° C. to prepare a honeycomb carrier coatedwith boehmite. Each of two types of the carriers was dipped into anaqueous solution containing 27 g/l of chloroplatinic acid. The productwas removed, dried, and then reduced at 200° C. for 1 hour in hydrogengas, to obtain the catalyst containing about 0.3% by weight of supportedplatinum. Air containing 500 ppm each of ethylene and methylchloride waspassed through the catalyst bed filled with the above catalyst on aspherical boehmite carrier at about 400° C. at a space velocity of39,000 hr.sup.⁻¹. The duration of reaction after initiation and thepresent decomposition of methylchloride and ethylene at thecorresponding times are shown in Table 3.

                  Table 3                                                         ______________________________________                                        Duration (hr.)                                                                              5      10      30    50    100                                  ______________________________________                                        Decomposition (%)                                                                          80      79      79    79    79                                   methylchloride                                                                ethylchloride                                                                              92      92      92    92    92                                   ______________________________________                                    

EXAMPLE 3

The oxidative decomposition of Example 2 was repeated except that thecatalyst was 0.3% by weight of platinum supported on a honeycomb carriercoated with boehmite and air containing ethylchloride instead ofmethylchloride was passed. The results are shown in Table 4.

                  Table 4                                                         ______________________________________                                        Duration (hr.)                                                                              5      20      50    100   200                                  ______________________________________                                        Decomposition (%)                                                                          93      93      93    93    93                                   ethylchloride                                                                 ethylene     96      96      96    96    96                                   ______________________________________                                    

EXAMPLE 4

A spherical boehmite carrier was dipped into an aqueous solution ofchloroplatinic acid. The product was removed, dried and then reducedwith an aqueous solution containing 2% NaBH₄ to prepare the catalystcontaining 0.3% by weight of platinum supported on the carrier. The airof Example 2 was passed through the catalyst bed filled with thecatalyst heated at 380° C. at a space velocity of 50,000 hr.sup.⁻¹. Theresults are shown in Table 5.

                  Table 5                                                         ______________________________________                                        Duration (hr.)  3        10        50                                         ______________________________________                                        Decomposition (%)                                                                            55        55        55                                         methylchloride                                                                ethylene       87        87        87                                         ______________________________________                                    

REFERENCE 2

The oxidative decomposition of Example 2 was repeated except that acatalyst prepared by supporting chloroplatinic acid on a commercialγ-alumina carrier to the extent of 0.3% by weight as Pt and thenreducing the acid with hydrogen was used. The results are shown in Table6.

                  Table 6                                                         ______________________________________                                        Duration (hr.)  3         5        10                                         ______________________________________                                        Decomposition (%)                                                                            55        52        48                                         methylchloride                                                                ethylene       87        85        83                                         ______________________________________                                    

Reference 3

A catalyst was prepared by supporting chloroplatinic acid on aα-aluminum honeycomb carrier to the extent of 0.3% by weight as Pt, andthen reducing the acid with hydrogen. The oxidative decomposition ofExample 3 was repeated substituting the above catalyst. The results areshown in Table 7.

                  Table 7                                                         ______________________________________                                        Duration (hr.)                                                                              3        5       10     20                                      ______________________________________                                        Decomposition (%)                                                                          60       62       53     45                                      ethylchloride                                                                 ethylene     98       98       96     93                                      ______________________________________                                    

Reference 4

A catalyst was prepared by supporting chloroplatinic acid on a mullite(3 Al₂ O₃ . 2SiO₂) honeycomb carrier to the extent of 0.3% by weight ofPt, and then reducing the acid with hydrogen. The oxidativedecomposition of Example 2 was repeated substituting the above catalyst.The results are shown in Table 8.

                  Table 8                                                         ______________________________________                                        Duration (hr.)   3           5                                                ______________________________________                                        Decomposition (%)                                                                              3           0                                                methylchloride                                                                ethylene         73          70                                               ______________________________________                                    

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the inventionas set forth herein.

What is claimed as new and intended to be covered by letters patentis:
 1. A method of oxidatively decomposing a chlorohydrocarbon havingfrom 1 to 10 carbon atoms to carbon dioxide, water, hydrogen chlorideand free chlorine which comprises contacting an exhaust gas containingfrom 10 - 10,000 ppm of said chlorohydrocarbon of from 1 - 10 carbonatoms and molecular oxygen with a catalytic amount of chromium oxidecatalyst at a temperature of from 200°-500° C.
 2. The process of claim1, wherein the catalyst contains 0.5 - 80% by weight of chromium oxidesupported on a carrier.
 3. The process of claim 1, wherein the catalystcontains 3 - 50% by weight of chromium oxide supported on a carrier. 4.The process of claim 1, wherein the space velocity of the gas is 5,000 -300,000 hr.sup.⁻¹.
 5. The process of claim 1, wherein thechlorohydrocarbon has from 1-2 carbon atoms.
 6. The process of claim 1wherein the chromium oxide catalyst is used in combination with aboehmite supported platinum catalyst.